Solar cell unit with removable layer

ABSTRACT

The invention pertains to a solar cell unit comprising a back electrode, a photovoltaic layer, a front electrode, and a transparent top layer, characterized in that on top of the top layer at least one removable transparent polymer film is present. Preferably, two to ten, more preferably three to five, removable transparent polymer films stacked one on top of the other are present on the top layer. On the bottom side the transparent polymer films may be provided with an adhesive layer that displays greater adhesion to the polymer film itself than to the underlying layer. Preferably, the solar cell unit is also provided with a substrate at the back of the back electrode. The presence of a transparent removable top layer enables easy removal of a surface once it has got dirty.

The present application is a national phase filing of PCT InternationalPatent Application No. PCT/EP01/06299, filed May 31, 2001 and claimspriority from Dutch Patent Application No. 1015449, filed on Jun. 15,2000.

The invention pertains to a solar cell unit with a removable top layer.

Solar cell units as a rule comprise a photovoltaic (PV) layer composedof a semiconductor material which is provided between a front electrode(at the front of the unit, i.e. on the side of the incident light) and aback electrode (at the back of the unit). The front electrode istransparent, enabling incident light to reach the semiconductormaterial, where the incident radiation is converted into electricenergy. In this way light can be used to generate electric power, whichoffers an interesting alternative to, say, fossil fuels or nuclearpower.

The top side of the solar cell unit, i.e. the side of the frontelectrode, is protected from the effects of its surroundings by atransparent top layer which comprises, e.g., sheet of glass or aplastics based sheet or layer. When the solar cell unit is in use, itstop layer will in due course get dirty, for instance because of soot,sand, and other dirt from the surrounding area being deposited thereon.As a result the transparency of the top layer will decrease, causing areduction in the yield of the solar cell unit. When the top layer ismade of glass, it can be cleaned in principle, but this is a labourintensive process, not least because solar cell units are often to befound on roofs or in otherwise poorly accessible places. Plastics basedtop layers generally are more fragile than top layers based on a glasssheet. This means that, on the one hand, the risk of damage to the toplayer from its surroundings, e.g., because of sand, is greater still,while, on the other hand, cleaning of the top layer is even moreproblematic. In other applications, such as portable applications intowhich solar cells have been integrated, it is conceivable that in duecourse the outer surface will be damaged by contact or will acquire aless attractive appearance in some other way. The result of said effectsis that it may be the top layer which in the end determines the lifespan of the solar cell unit, even though, generally speaking, it is notthe most expensive part of solar cell unit.

For that reason there is need for a system with which labour intensivecleaning of the solar cell unit can be avoided and, more particularly inthe case of plastics based top layers, the life span of the solar cellunit can be extended.

The present invention has for its object to resolve these problems andto this end provides a solar cell unit comprising a back electrode, aphotovoltaic layer, a front electrode, and a transparent top layer,characterised in that on top of the top layer at least one removabletransparent polymer film is present. When during use the solar cell unitaccording to the invention gets dirty or is damaged, so that thetransparency of its surface is reduced to an unacceptable level, the toppolymer film is simply removed, giving a fresh surface which is cleanand undamaged. This process can be repeated as many times as there areremovable transparent polymer films available.

In the context of the present specification the term top layer refers toa stable top layer, that is, a top layer of which the internal physicaland chemical properties, e.g., the degree of polymerisation, are notcapable of changing substantially during use.

The number of removable transparent polymer films is determined by theconditions in question. With a too high number of transparent polymerfilms, the quantity of light reaching the active layers of the solarcell unit through the transparent polymer films becomes too low. Whenthe number of transparent polymer films is too low, the solar cell unitsurface cannot be renewed often enough, as a result of which the effectof the invention is insufficiently achieved. Preferably, there are 2-10removable transparent polymer films stacked one on top of the other onthe top layer, more preferably 3-5 films. The number of removabletransparent polymer films will also depend on the use to which the solarcell unit is ultimately put. In the case of solar cell units employed inhighly polluting surroundings, e.g., alongside motorways, a highernumber of removable transparent polymer films will be opted for thanwhen the solar cell units are employed in less polluting surroundings.

The Drawings further illustrate the present invention wherein:

FIG. 1 is a cross sectional view of a solar cell according to the priorart; and

FIG. 2 is a cross sectional view of an embodiment of a solar cellaccording to the present invention.

The invention will be elucidated further with references to FIGS. 1 and2. FIG. 1 shows a well-known solar cell unit wherein (1) represents aphotovoltaic layer, (2) represents the back electrode, (3) representsthe front electrode, and (4) is the transparent top layer based on,e.g., glass or plastic (5) stands for an optionally present substrateserving as a carrier or protective layer. FIG. 2 shows an example of asolar cell unit according to the invention, where on top of the solarcell unit of FIG. 1 a stack (6) of 3 removable transparent polymer filmshas been provided.

In the embodiment illustrated in FIG. 2, each polymer film is providedon the bottom side with an adhesive layer which displays greateradhesion to the polymer film itself than to the underlying layer, sothat when the polymer film is removed, the adhesive layer is removedalong with the film. Depending on the nature of the polymer film, e.g.,its static properties, the presence of an adhesive layer is not alwaysessential, however.

The present invention also pertains to a process for generatingelectricity using a solar cell unit of the present invention. In thisprocess a solar cell unit according to the invention comprising a toplayer and at least one removable transparent polymer film is exposed tothe environment. When the transparency of the polymer film present atthe top of the solar cell unit has decreased to an unacceptable level,e.g., by deposition of dirt, the top polymer film is removed, exposingthe polymer film below it to the environment. When more than one polymerfilm is present, this step is repeated until all polymer films have beenremoved. Then, the top layer itself is exposed to the environment.

Suitable materials for the removable transparent polymer films are knownas such. They comprise, int. al., films based on polyesters, polyamides,polyimides, polyether sulphones, polycarbonates, polyolefins, acrylates,and acetates.

Suitable examples include films of polyethylene, polypropylene,polyester, cellulose triacetate, polycarbonate, and polyamide. Polymerfilms provided with transparent barrier layers to counter hard UV raysand gases and liquids are also emcompassed by the invention, as areanti-fouling layers.

The polymer film generally has a thickness of 5 to 60 μm, preferably of10 to 25 μm.

Needless to say, the adhesive, if present, has to be transparent. Forthe rest, use may be made of suitable types of adhesives known to theskilled person. In the present specification the term transparent hasthe meaning of light-transmitting in the frequency range where light isconverted into electric energy by the photovoltaic layer.

When selecting the material for the removable transparent polymer filmand determining its thickness, the following should be taken intoaccount. On the one hand, it is desired to keep the adsorption of lightby the polymer films as low as possible, in order for the quantity oflight reaching the active layers of the solar cell unit to remain ashigh as possible. On the other hand, the polymer film has to have suchintrinsic strength as will enable its easy removal. Taking the aboveinto account, the skilled person will be able to select a suitableremovable transparent polymer film.

Depending on the size of the solar cell unit, it may be that theremovable transparent polymer film will be composed of a number ofadjacent sections. In that case it is preferred to ensure that in theseveral polymer films composed of a number of sections which are stackedone on top of the other, each film has its transitions between thevarious sections provided in different places from the transitions inthe films situated immediately above and underneath it, this to preventa number of films being removed at the same time.

The polymer films and any adhesive layers present can be provided on thesolar cell unit in manners known to the skilled person, e.g., bylaminating, brushing, etc. The nature of the solar cell unit which isprovided according to the invention with at least one removabletransparent polymer film is not critical.

The invention may be applied to rigid solar cell units, e.g., solar cellunits where the top layer is composed of a sheet of glass. Examples ofthis are the well-known solar cell units based on crystalline silicon.

However, as was indicated earlier, the invention is especiallyattractive for use with solar cell units with a plastic top layer, sincegenerally speaking such solar cell units suffer more from damage andfouling than solar cell units with a top layer made of glass do. Solarcell units with a plastic top layer comprise rigid solar cell unitsbased on, e.g., a ceramic carrier, but especially flexible solar cellunits based on a carrier of, e.g., metal or a plastic material. Theinvention is especially suited to be used in solar cell units made bymeans of a continuous process, more particularly, in flexible solar cellunits made in a roll-to-roll process, e.g., as described in WO 98/13882or WO 99/49483. Said publications are incorporated by reference into thepresent description as regards the process for manufacturing theflexible solar cell units and the materials used in said process.

As indicated above, a solar cell unit comprises a back electrode, aphotovoltaic layer, and a front electrode, as well as a transparent toplayer. Depending on the conditions, the solar cell unit will alsocomprise, at the back of the back electrode, a substrate serving as acarrier or protective layer. The nature of the carrier, the backelectrode, the photovoltaic layer, and the front electrode is notcritical to the present invention. The following description is offeredmerely by way of illustration.

The transparent top layer may be any well-known transparent top layerand comprises, e,g., a sheet of glass or a plastic sheet or film. Thefront electrode generally is a transparent conductive oxide (TCO).Suitable TCOs comprise indium tin oxide, zinc oxide, aluminium-,fluorine- or boron-doped zinc oxide, cadmium sulphide, cadmium oxide,tin oxide, and F-doped SnO₂.

The photovoltaic layer may comprise any suitable system known to theskilled person, e.g., amorphous silicon (a-Si:H), microcrystallinesilicon, polycrystalline silicon, amorphous silicon carbide (a-SiC) anda-SiC:H, amorphous silicon-germanium (a-SiGe) and a-SiGe:H, a-SiSn:H,CIS (copper indium diselenide, CuInSe₂), cadmium telluride, Cu(In,Ga)Se,ZnSe/CIS, ZnO/CIS, Mo/CIS/CdS/ZnO, and dye-sensitised cells. Also,stacked cells of the aforesaid materials may be employed.

The back electrode, which depending on the use to which the solar cellunit is put may also serve as reflector, may be made up of, e.g.,aluminium, silver or a combination of the two, optionally with anintermediate layer of a dielectric such as ZnO arranged between themetal layer and the photoactive semiconductor. Depending on the natureof the system, the substrate optionally present at the back of the backelectrode may serve as a carrier or only have a protective function. Ifso desired, the substrate may be transparent when the solar cell unithas to be wholly or partially transparent. (In the latter case, the backelectrode of course also should be wholly or partially transparent.)Suitable materials are determined by the function of the substrate. Whenthe substrate has a carrier function, it may be, e.g., a rigid orflexible supporting material, such as a sheet or foil of plastic ormetal. When the substrate primarily has a protective function, it maybe, e.g., a thin plastic film.

What is claimed is:
 1. A solar cell unit comprising a back electrode, aphotovoltaic layer, a front electrode, and a transparent top layer,wherein on top of the top layer two to ten removable transparent polymerfilms, stacked one on top of the other, are present.
 2. The solar cellunit of claim 1 wherein three to ten removable transparent polymerfilms, stacked one on top of the other, are present on the top layer. 3.The solar cell unit of claim 1 wherein three to five removabletransparent polymer films, stacked one on top of the other, are presenton the top layer.
 4. The solar cell unit of any one of claims 1-3wherein each of the transparent polymer films are provided at theirrespective bottom side with an adhesive that displays greater adhesionto the polymer film itself than to the underlying layer.
 5. The solarcell unit of any one of claims 1-3 which is additionally provided with asubstrate at the back of the back electrode.
 6. A method for generatingelectricity using a solar cell unit comprising a back electrode, aphotovoltaic layer, a front electrode, and a transparent top layer, andon top of the top layer a removable transparent polymer film, or a stackof removable transparent polymer films, wherein the transparent polymerfilm or the top film of the stack of removable transparent polymer filmsis exposed to the environment, wherein when the transparency of thepolymer film present at the top of the solar cell unit has decreased toan unacceptable level, the polymer film or the top polymer film isremoved, this step being repeated until all polymer films have beenremoved, where after the top layer itself is exposed to the environment.7. A method for cleaning a solar call unit comprising a back electrode,a photovoltaic layer, a front electrode, and a transparent top layer,and on top of the top layer a removable transparent polymer film or astack of removable transparent polymer films, comprising the step ofremoving the transparent polymer film er the top film of the stack ofremovable transparent polymer films when the transparency of the polymerfilm present at the top of the solar cull unit has decreased to anunacceptable level.
 8. The solar cull unit of claim 4 which isadditionally provided with a substrate at the back of the backelectrode.